The invention relates to a drive system for a motor of which the speed is controlled.
FIG. 1 is a conventional motor drive system for controlling the motor speed. In the figure, reference numeral 1 designates a frequency-voltage converter; 2 an error signal amplifier; 3 a reference power source; 4 an emitter follower transistor for current amplification; 5 a motor; 6 a motor speed sensor for the motor. In operation, the motor speed of the motor 5 is detected by the motor speed sensor 6 which applies a signal with a frequency proportional to the motor speed to the frequency-voltage converter 1. The output signal from the frequency-voltage converter 1 is compared with the voltage Vr of a reference power source 3 such as a D.C. power source by the error signal amplifier 2 and the error signal amplifier 2 produces a DC voltage proportional to a signal representing difference therebetween. The DC voltage is applied to the emitter follower transistor 4 of which the output is applied to the motor 5. The number of revolutions of the motor is converted into a DC voltage by the frequency-voltage converter 1 and the DC voltage converted is constantly compared with the voltage Vr of the reference power source 3, so that it operates so as to make the difference voltage smaller and thus the motor speed is made constant. More specifically, when the load of the motor increases and the number of the rotations of the motor reduces, the frequency of the output signal from the motor speed sensor 6 also reduces, and the output signal from the frequency-voltage converter 1 has a large difference from the voltage Vr of the reference power source 3, so that the output signal from the error difference signal amplifier 2 increases. As a result, the output voltage of the emitter follower transistor 4 increases to increase the number of the revolutions of the motor, whereby the motor speed is kept constant.
With such a construction, a voltage applied to the motor at the stationary torque may be small, but it is necessary to increase the application voltage to the motor with the increase of the load torque of the motor. Therefore, it is necessary to increase a voltage drop across the emitter follower transistor 4 at the normal load by a voltage corresponding to the increasing motor voltage applied at the increase of the load. This increased voltage drop is consumed as a power loss. The increase of the load torque of the motor takes place not only at the start of the motor but also due to scar or water drop on a video tape in the case of a video tape recorder. In order to keep the motor speed constant against this frequent load variation, a good ability of the motor speed control must be secured and a start time of the motor must be reduced, so that high power source voltage is required and, at a normal load condition, the pregiven voltage is consumed by the emitter follower transistor 4 to drive the motor at a low voltage.
The explanation will be given about the characteristics between a load torque .tau. and N (hereinafter referred to .tau.-N characteristic) of a general DC motor. The .tau.-N characteristic of the general DC motor is illustrated in FIG. 2 with a parameter of the application voltage Vcc. As indicated by lines 8 to 11 in FIG. 2, the motor speed proportionally reduces with increase of the load. Therefore, the motor application voltage must be increased with the increase of the load so that the motor speed is not reduced even if the load is increased. Specifically, in order to obtain the motor speed No at no load (.tau..sub.0 =0), the motor application voltage may be Vcco. In order to keep the motor speed No constant regardless of increase of the load torque to .tau..sub.1, however, it is necessary to increase the output voltage from the emitter follower transistor 4 and to increase the motor application voltage to Vcc1. Further, as the load torque increases to .tau..sub.2 and .tau..sub.3, the application voltage must be increased to Vcc2 and Vcc3 respectively. If the motor application voltage Vcc3 is the voltage of the power source 7, the motor application voltage is not increased beyond it, so that the circuit can not control the motor speed at constant against the load torque larger than .tau..sub.3 and thus the motor speed is reduced. Therefore, a sufficient voltage must be needed in order to stably keep the motor speed against such a load variation.
Accordingly, a voltage obtained by subtracting a motor voltage applied at the stationary load from the motor application voltage Vcc, is applied to a transistor 4, so that there occurs a problem that a power loss corresponding to the product of the difference voltage and the current caused by the voltage is always produced.